This invention relates to a packing element for a tower. The element is of a new and useful shape. The element, when poured randomly in a tower or column to form a packed bed, produces a large effective surface for a gas-liquid operation and a liquid-liquid operation such as, gas absorption, distillation, liquid extraction, chemical reaction and other similar operations that employ mass and heat transfer.
These operations are conducted with the light liquid normally flowing upwardly through a packed bed of packing elements and the heavy liquid flowing downwardly opposite to the flow of the light fluid. The heavy fluid always flows downwardly and is driven by gravity. In some cases the light fluid rises due to bouyant forces as in liquid-liquid extraction. In gas-absorption the light fluid can flow in the same direction or perpendicular to the heavy fluid flow. The pressure differentials across the packed bed drive the light fluid through the bed.
The most common packings are of two general types: surface type packing and filament type packing. The surface type packing produces a more serpentine path for the fluids as they pass through the bed than does the filament type packing. This results in higher pressure drops for surface type packing and better distribution of fluids than does the filament type packing.
The main categories of surface type packing are rings (as disclosed in Raschig U.S. Pat. No. 1,141,266, and Eckert U.S. Pat. No. 3,266,787) and saddles (as disclosed in Berl U.S. Pat. No. 1,796,501, Leva U.S. Pat. No. 2,639,909.)
The filament type packing includes toroidal and spherical packets of filaments. The toroidal filament packet was disclosed initially in Teller U.S. Pat. No. 2,867,425 and then in Doyne U.S. Pat. No. 3,752,453. The spherical filament packet was disclosed in Fattinger U.S. Pat. No. 4,072,736 and Kackenjos, U.S. Pat. No. 4,203,935.
The surface type packing has heavy fluid cascading over its upper surface driven by gravity and has heavy fluid dispersed over its underside surface using the velocity head of the light fluid. A relatively high pressure drop in light fluid is necessary for good wetting of this type of packing. A lack of symmetry in these packing shapes causes local areas of varying void densities within the packed bed. This creates a preferential path of flow and results in poor contact between the heavy fluid and the light fluid. The curved surfaces of surface type packings provide sites for stagnant heavy fluids to be caught within the packed bed. An extremely high pressure drop across the packed bed can occur during high loading thereby forcing the light fluid to by-pass the packed bed at the wall of the tower or column. In this situation the packed bed is not functioning as designed as it is not effective.
Filament type packing does not distribute the heavy or light fluid due to its geometry. With filament type packing heavy fluid trickles over the filaments coating the total surface (if the heavy fluid is distributed well initially over the bed's cross section.) In this circumstance good wetting of the surface of the filament at low pressure drops is possible. To produce good interfacial contact both heavy and light fluids must be well distributed initially. The pressure drop that is necessary within a packed bed for distillation cannot be adequately achieved with filament type packing. This results in a restriction of the use of filament type packing.
A packed bed settles and results in a smaller volume as compared to the initial volume of the bed. Both surface type packing and filament type packing produce beds that settle. Bed stability and resistance to nesting depends on packing geometry and the configuration of the packing.